Method of producing gypsum

ABSTRACT

After hydrogenation and desulfurization treatment of raw fuel in the desulfurization unit  1 , the product is separated in the acidic gas separator  2  into fuel and a hydrogen sulfide-containing gas, and the hydrogen sulfide-containing gas is subjected to combustion together with air in the catalyst converter  3  thereby converting the hydrogen sulfide completely into sulfur dioxide to give a sulfur dioxide-containing gas, and this sulfur dioxide-containing gas is reacted with limestone powder and air in water in the oxidation and neutralization reactor  4 , and the resulting slurry is dehydrated in the gypsum slurry solid/liquid separator  5  and then dried in the gypsum heater  6.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of producing gypsum byuse of a hydrogen sulfide-containing gas formed on the desulfurizationof raw fuel such as crude oil, heavy oil, coal.

[0003] 2. Description of the Related Art

[0004] Hydrogen sulfide occurring as by-product by desulfurization ofraw fuel such as crude oil, heavy oil, coal is utilized again afterconverting it into sulfur, sulfuric acid, sodium sulfate, sodiumsulfite, magnesium sulfate, calcium sulfate and so on, or is discarded.The amount of recovered sulfur is increasing ever year and this isproblematic. The sulfur is used predominantly for production of sulfuricacid, further as a catalyst in the form of sulfur dioxide, chemicallyformed sulfur or sodium sulfite, and for synthetic fiber, rubber,treatment of pulp, inorganic chemicals, detergents, chemical productsand so on. Calcium sulfate is used in a large amount as gypsum inadditives for building materials and cement. The demand for gypsum inJapan has reached 9.5 million tons/year (1.77 million tons/year in termsof sulfur), and 4 million tons/year (0.7 million ton/year in terms ofsulfur) was imported into Japan and its demand is expected to beincreasing as well from now on.

[0005] When raw fuel such as crude oil, heavy oil or coal is gasifiedand desulfurized, the resulting hydrogen sulfide will containby-products such as carbon dioxide. Therefore, if the hydrogen sulfideis separated with a basic adsorbent from the fuel, hydrogen sulfidecontaining by-products such as carbon dioxide is recovered. Forproduction of gypsum, therefore, it becomes necessary to use thishydrogen sulfide containing carbon dioxide, that is, a hydrogensulfide-containing gas, as the starting material.

[0006] Under these circumstances, Japanese Patent ProvisionalPublication 237,921/1995 discloses that after a gas containing sulfurdioxide in high concentration is obtained by combustion of hydrogensulfite separated from raw fuel, said gas containing sulfur dioxide inhigh concentration is converted into gypsum slurry by contacting saidgas with water slurry and simultaneously supplying calcium carbonate andair to the water slurry, and gypsum is separated as a cake from saidgypsum slurry, and this cake is dried using as a heat source thecombustion heat generated in the combustion of the hydrogen sulfide.

[0007] However, the above Japanese Patent Provisional Publication237,921/1995 does not disclose that hydrogen sulfide in the range of lowto high concentrations, obtained by hydrogenation and desulfurizationtreatment or partial oxidation treatment of raw fuel and subsequentseparation from the fuel, is completely oxidized to produce gypsum.

SUMMARY OF THE INVENTION

[0008] The object of the present invention is to provide high-puritygypsum effectively by completely oxidizing hydrogen sulfide in a widerange of concentration, obtained by hydrogenation and desulfurizationtreatment or partial oxidization treatment of raw fuel and subsequentseparation from the fuel.

[0009] To achieve this object, the present invention relates to a methodof producing gypsum, which comprises the steps of: separating a hydrogensulfide-containing gas from raw fuel after hydrogenation anddesulfurization treatment or partial oxidation treatment of said rawfuel; subjecting said hydrogen sulfide-containing gas to combustion witha catalyst and/or a combustion improver to produce a sulfurdioxide-containing gas; and reacting said sulfur dioxide-containing gaswith oxygen and calcium carbonate under wet conditions to producegypsum.

[0010] In the above method of producing gypsum, said catalyst ispreferably an oxide of at least one element among Cu, Cr, Fe, Ni, Co,Mn, La, Ba, and Ca.

[0011] In the above method of producing gypsum, the combustion heat ofsaid hydrogen sulfide-containing gas can be recovered as steam or warmwater.

[0012] In the above method of producing gypsum, the combustion heat ofsaid hydrogen sulfide-containing gas can be used as a heat source fordrying said gypsum and/or an energy source for generation ofelectricity.

[0013] In the method of producing gypsum according to the presentinvention, a hydrogen sulfide-containing gas even with a lowconcentration of hydrogen sulfide can be used to give a sulfurdioxide-containing gas free of hydrogen sulfide and so forth bysubjecting the hydrogen sulfide to complete combustion, so high-puritygypsum can be efficiently obtained by reacting said sulfurdioxide-containing gas with oxygen and calcium carbonate under wetconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 illustrates one embodiment of the method of producinggypsum according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, the embodiments for carrying out the method ofproducing gypsum according to the present invention are described byreference to FIG. 1.

[0016] The raw fuel used in the present invention includes raw oil,heavy oil, coal, or imulsion, oil sand, oil slurry and so on.Desulfurization treatment means the conversion of the sulfur content inraw fuel into hydrogen sulfide. For this, raw fuel is hydrogenated anddesulfurized, for example, in a desulfurization unit 1, or partiallyoxidized at high temperature. Although in partial oxidization, thecarbon content in raw fuel is oxidized into carbon monoxide or carbondioxide, the sulfur content is converted into hydrogen sulfide withoutforming any sulfur oxide because this reaction is carried out in areducing atmosphere. The raw fuel is subjected to hydrogenation anddesulfurization treatment or partial oxidization treatment, and thenseparated into a hydrogen sulfide-containing gas (i.e. hydrogen sulfitecontaining carbon dioxide etc.) and desulfurized fuel in the acidic gasseparator 2 provided with an adsorbent and so forth for selectiveadsorption of acidic gas. This separation can be carried out using aselective adsorbent such as basic solvent for acidic gas.

[0017] Said hydrogen sulfide-containing gas is mixed with oxygen(including an air atmosphere in some cases) and oxidized in a combustionfurnace etc. to form sulfur dioxide (SO₂). The formed sulfur dioxide isabsorbed into water to form sulfurous acid (H₂SO₃) and it is furtheroxidized into sulfuric acid (H₂SO₄) by introducing oxygen into thiswater. The sulfuric acid reacts with calcium carbonate (CaCO₃) dispersedin water to form calcium sulfate (CaSO₄), i.e. gypsum. However, ifoxidation in said desulfurization treatment is insufficient and hydrogensulfide or sulfur still remains, a sulfurous acid compound or athiosulfuric acid compound is formed to cause lower purity, and by theseimpurities the rate of formation of gypsum is lowered. Particularly in ahydrogen sulfide-containing gas obtained by desulfurization of raw fuel,the hydrogen sulfide content is low because of carbon dioxide and so onmixed in it, and therefore, inadequate oxidation readily occurs and theabove-described problem is easily brought about.

[0018] Accordingly, a method by completely oxidizing hydrogen sulfide ina wide range of concentration into sulfur dioxide to produce gypsum wasestablished in the present invention. Specifically, a gas containinghydrogen sulfide in a wide range of concentration, obtained byhydrogenation and desulfurization treatment or partial oxidation of rawfuel and subsequent separation from the fuel, is oxidized with acatalyst and/or a combustion improver until no hydrogen sulfide orsulfur remains. That is, hydrogen sulfide in a low to high concentrationin said hydrogen sulfide-containing gas is converted into sulfur dioxideby complete oxidation with a catalyst in a catalyst-layer converter 3.The concentration of hydrogen sulfide present in the hydrogensulfide-containing gas suffices at 5% or more, preferably 10% or more.

[0019] The catalyst used in oxidation of hydrogen sulfide includes anoxide of at least one element among Cu, Cr, Fe, Ni, Co, Mn, La, Ba, andCa. Such catalyst can be carried in a carrier for use. The carrierincludes silica, alumina, zirconia, cojelite, mullite and so on. Theshape of carrier includes granular or honeycomb structure as examples.The catalyst is charged into the catalyst-layer converter 3 forcombustion of the hydrogen sulfide-containing gas.

[0020] In addition, said hydrogen sulfide-containing gas can besubjected to combustion by addition of a combustion improver to covertit completely into sulfur dioxide. If necessary, said hydrogensulfide-containing gas is subjected to combustion by addition of acombustion improver, and the resulting gas can be further passed througha catalyst layer to be completely converted into sulfur dioxide. Thecombustion improver includes combustible gases such as methane, ethane,propane, butane, and city gas. The combustion improver may be blown intothe catalyst layer for combustion.

[0021] The gas discharged from the outlet of the catalyst-layerconverter 3 contains sulfur dioxide resulting from the completeoxidization of hydrogen sulfide and sulfur. The gas from the outlet ofthis converter 3, that is, a sulfur dioxide-containing gas, is fed to anoxidization and neutralization reactor 4 via a gas blowing tube havingan outlet below the surface of the water, and the gas is dispersed andabsorbed into the water. For this dispersion, devices for generatingfine gas bubbles, such as stationary disperser, arm-rotating disperser,rotary atomizer, are preferably used. To ensure sufficient absorption ofthe sulfur dioxide-containing gas and its adequate reaction with oxygenand further adequate reaction with calcium carbonate, the outlet for thesulfur-dioxide-containing gas is located preferably in a deeperposition, for example 1 meter or more deeper.

[0022] In a gaseous phase, sulfur dioxide (SO₂) is gradually oxidized byoxygen into sulfur trioxide (SO₃), but in an aqueous solution, theoxidization of sulfurous acid into sulfuric acid proceeds instantly.However, because the rate of dissolution of oxygen in the gaseous phaseinto water is rate-limiting, the sulfurous acid can be oxidizedcompletely into sulfuric acid by dispersing an excess of anoxygen-containing gas in water as fine gas bubbles. Oxygen for theoxidation is generally air, and together with the gas from thecombustion step, air is dispersed in the water in the oxidation andneutralization reactor 4 via an air-blowing tube.

[0023] The water in the reactor 4 is made strongly acidic due tosulfurous acid occurring by absorption and sulfuric acid formed byoxidation. This strongly acidic water prevents a sulfuric acid gas frombeing absorbed thereinto, so it should be neutralized with an alkalinesubstance. By using calcium carbonate as the alkaline substance in thepresent invention, calcium sulfate is formed and recovered as gypsum.

[0024] If the sulfur dioxide-containing gas has contained hydrogensulfide and sulfur, the following side reaction occurs, and theoxidation of sulfurous acid into sulfuric acid is inhibited by theformed thiosulfuric acid and so on, resulting in a slow rate offormation of gypsum and a decrease in purity of gypsum.

2H₂S+4SO₃ ²⁻+2H⁺→3S₂O₃ ²⁻+3H₂O

S+SO₃ ²⁻→S₂O₃ ²⁻

[0025] The most economical source of calcium carbonate is limestone.However, calcium oxide and calcium hydroxide can be also used as aneutralizing agent to obtain gypsum. In addition to limestone, calciteand marble can also be used, and a low concentration of lime can be usedin some cases for cement. Limestone is preferably used in the form ofpowder, and it can be directly supplied to the reactor 4 or as slurryafter dispersed in water.

[0026] The powder of the limestone is dissolved while reacting withsulfuric acid and precipitated as crystals of lime dihydrate(CaSO₄·2H₂O). The gypsum crystals are plate- and column-shaped and about100 μm in size. As the crystals are formed, the water in the reactor 4becomes slurry having crystals of gypsum suspended in it. The amount oflimestone supplied is determined depending on the amount of a sulfurousacid gas generated. Usually, the pH in the reactor 4 is detected andlimestone is introduced such that the pH is maintained in the weaklyacidic range of pH 2 to 7.

[0027] In this manner, the sulfurous acid gas is absorbed into water,oxidized, and neutralized with limestone, and a sulfurous acid-free gasis exhausted from the oxidation and neutralization reactor 4.

[0028] The amount of gypsum in the reactor 4 is increased with anincreasing absorption amount of a sulfurous acid gas, and theconcentration of slurry as gypsum is increased. When the concentrationof suspended gypsum exceeds 30% by weight, the fluidity of the slurry isdecreased and stirring is difficult, and thus part of the slurry isremoved and sent to the step of separating gypsum. The amount of thedecreased liquid in the reactor 4 is regulated by supplying water. Thegypsum slurry removed from the reactor 4 is transferred by e.g. a slurrypump to a gypsum slurry solid/liquid separator 5 where it separated intoa gypsum cake and a supernatant by a solid/liquid separation unit suchas centrifuge. The supernatant is returned to the reactor 4 as a part ofwater supplied to the reaction step.

[0029] The separated gypsum cake usually contains about 7% water byweight, so it is conveyed to the gypsum heater 6 by a belt conveyer andso on. In the drying step, water of hydration is evaporated by a heatersuch as drying furnace or a calcination furnace. The gypsum becomesgypsum dehydrate (CaSO₄·2H₂O), gypsum hemihydrate (CaSO₄·½H₂O), orgypsum anhydride (CaSO₄), depending on degree of heating. The gypsumdihydrate becomes gypsum hemihydrate by heating at 120-150° C. Thegypsum hemihydrate is called β-type gypsum hemihydrate or calcinedgypsum, and it is obtained with high purity of at least 95 % by weight,so it can be used by itself a starting material for production ofplaster board for building materials.

[0030] As the heat source of the above-described gypsum heater 6, thecombustion heat of said hydrogen sulfide-containing gas can be utilized.The reaction heat of hydrogen sulfide is 3647 kcal/kg (or 124 kcal/molof sulfur), and can be effectively utilized as the heating source. Incase methane is used as the combustion improver, additional sufficientheat can be recovered and utilized. The means for transmitting the abovecombustion heat of said hydrogen sulfide-containing gas to the gypsumheater 6 includes a method of recovering the heat as steam or warm waterand a method of directly introducing the gas in the combustion step intoa heating unit such as a calcination oven or a kiln. The abovecombustion heat can also be used as an energy source for generation ofelectricity.

[0031] The present invention can be effected in any of batch, semi-batchand continuous systems.

(Example)

[0032] The present invention is described below by reference to thefollowing examples.

Example 1

[0033] As the raw fuel, or imulsion was used and desulfurized under wetconditions, and it was separated into a hydrogen sulfide-containing gasand fuel by means of an amine-type adsorbent. The composition of theresulting hydrogen sulfide-containing gas is shown in Table 1. Forcombustion of the hydrogen sulfide-containing gas, a catalyst in whichoxide iron had been carried on a carrier consisting of honeycomb-shapedalumina was introduced into a combustion converter, and air was suppliedin the ratio shown in Table 1, and the hydrogen sulfide-containing gaswas subjected to combustion at a temperature of about 560°C. to give asulfur dioxide-containing gas. The composition of this sulfurdioxide-containing gas is shown in Table 1. TABLE 1 Hydrogen sulfide-Combustion Sulfur dioxide- containing gas air containing gas vol %kgmol/h kgmol/h vol % kgmol/h H₂S 13.70 173.0 0.00 0.0 SO₂ 0.00 0.0 5.67173.0 S trace trace 0.00 0.0 CO₂ 80.50 1016.7 33.41 1019.3 CO 0.20 2.50.00 0.0 H₂0 5.38 67.9 7.99 243.8 H₂ 0.22 2.8 0.00 0.0 CH₄ 0.00 0.0 0.000.0 N₂ 0.00 0.0 1563.8 51.27 1563.8 O₂ 0.00 0.0 312.8 1.66 50.5 Total100 1263.0 1876.6 100 3050.4 Total 28291.2 42035.3 68330.0 (m³N/h)

[0034] As can be seen from Table 1, the concentration of hydrogensulfide in the hydrogen sulfide-containing gas is as low as about 14%.The sulfur dioxide-containing gas obtained by combustion of the hydrogensulfide-containing gas did not contain hydrogen sulfide or sulfurbecause they were oxidized completely into sulfur dioxide.

[0035] Then, the sulfur dioxide-containing gas was absorbed into water,oxidized into sulfuric acid by further blowing air, and neutralized withlimestone powder, whereby gypsum was produced. When air was blown for 1hour in the above step, the sulfur dioxide was converted completely intogypsum, and it was not possible to detect thiosulfite and so on as asubstance inhibiting oxidation into sulfuric acid. The purity of thegypsum thus obtained was 96% by weight.

[0036] To confirm the effect of the present invention, ComparativeExample was carried out as follows:

Comparative Example 1

[0037] As the raw fuel, or imulsion was used and desulfurized under wetconditions, and it was- separated into a hydrogen sulfide-containing gasand fuel by means of an amine-type adsorbent. The composition of theresulting hydrogen sulfide-containing gas is shown in Table 2. Thishydrogen sulfide-containing gas was subjected to combustion using onlyair. The volume of air supplied is shown in Table 2. The hydrogensulfide-containing gas was subjected to combustion at a combustiontemperature of about 780° C. The composition of the sulfurdioxide-containing gas obtained by combustion is shown in Table 2. TABLE2 Hydrogen sulfide- Combustion Sulfur dioxide- containing gas air, etccontaining gas vol % kgmol/h kgmol/h vol % kgmol/h H₂S 13.70 173.0 0.6819.6 SO₂ 0.00 0.0 5.35 153.5 S trace trace trace trace CO₂ 80.50 1016.735.50 1019.3 CO 0.20 2.5 0.00 0.0 H₂O 5.38 67.9 7.81 224.3 H₂ 0.22 2.80.00 0.0 CH₄ 0.00 0.0 0.00 0.0 N₂ 0.00 0.0 1563.8 48.97 1406.0 O₂ 0.000.0 312.8 1.68 48.3 Total 100 1263.0 1876.6 100 2870.8 Total 28291.242035.3 64305.9 (m³N/h)

[0038] As can be seen from Table 2, the concentration of hydrogensulfide in the hydrogen sulfide-containing gas was as low as about 14%,and hydrogen sulfide and sulfur were not completely oxidized and theyremained in the sulfur dioxide-containing gas obtained by combustion ofthe hydrogen sulfide-containing gas with air.

[0039] Then, this sulfur dioxide-containing gas was absorbed into water,then oxidized into sulfuric acid by blowing air, and neutralized withlimestone powder, whereby gypsum was produced. At this time, thiosulfateions were formed from the remaining hydrogen sulfide and sulfur, andeven if air was passed through the water for 1 hour, the concentrationof sulfite ions in the slurry was high, and its oxidation into sulfuricacid was inadequate, so the rate of formation of gypsum wassignificantly lowered.

1. A method of producing gypsum, which comprises the steps of:separating a hydrogen sulfide-containing gas from raw fuel afterhydrogenation and desulfurization treatment or partial oxidationtreatment of said raw fuel; subjecting said hydrogen sulfide-containinggas to combustion with a catalyst and/or a combustion improver toproduce a sulfur dioxide-containing gas; and reacting said sulfurdioxide-containing gas with oxygen and calcium carbonate under wetconditions to produce gypsum.
 2. A method of producing gypsum accordingto claim 1 , wherein said catalyst is an oxide of at least one elementamong Cu, Cr, Fe, Ni, Co, Mn, La, Ba, and Ca.
 3. A method of producinggypsum according to claim 1 or 2 , further comprising the step ofrecovering combustion heat of said hydrogen sulfide-containing gas assteam or warm water.
 4. A method of producing gypsum according to anyone of claims 1-3, further comprising the step using the combustion heatof said hydrogen sulfide-containing gas as a heat source for drying saidgypsum and/or an energy source for generation of electricity.